1. Field of the Invention
This invention relates generally to the field of battery chargers with overcharge protection circuits. More particularly, this invention relates to a charging system in which a battery module is charged through a host system and wherein redundant overcharging protection is provided in the host system as well as the battery module.
2. Background
Lithium ion batteries have become very popular in recent years due to their high energy density, high terminal voltage and lack of a memory effect which is common in other rechargeable batteries such as nickel cadmium. While lithium ion batteries have many advantages, they must be properly charged in order for the user to obtain full benefit of these batteries. For this reason, safeguards have been taken to assure that lithium ion batteries are carefully charged and include overcharge protection such as that described in U.S. Pat. No. 5,493,197 to Eguchi et al. and U.S. Pat. No. 5,530,336, also to Eguchi et al. Examples of overcharge protection circuits may be found in U.S. Pat. No. 5,304,915 to Sampei et al. and PCT application Serial Number PCT/JP 93/01536 to Masaru et al. Each of the above documents are hereby incorporated by reference. However, an additional problem exists when the battery module has been damaged, for example, by being dropped. The protective switch and associated circuits in the battery module could become inoperative and therefore, the battery module would lose overcharge protection.
The present invention addresses the problem by providing a strong measure of redundancy in a battery charging system.